FIGS. 1 and 2A disclose a front view and a perspective view of a traditional floating SPAR platform 100 for offshore oil drilling and production. In FIG. 2A, the SPAR platform 100 includes a topside facility 202 for oil drilling and production and a draft hull 204. The topside facility 202 is located above a waterline 200. The draft hull 204 is located below the topside facility 202 and is normally 650 to 750 feet long, with a 90 to 160 feet diameter, and mostly submerged in the seawater. The draft hull 204 usually has an in-service draft in the order of 600 to 700 feet depending on metocean conditions of the area where the SPAR platform 100 is deployed.
The draft hull 204 can include three main components: a hard tank 206 located in the upper part of the draft hull 204 for providing buoyancy to support the topside facility 202, a mid tank 208 located below the hard tank 206 for oil storage or completely being flooded with seawater, and a soft tank 210 located at the bottom of the SPAR platform 100 for providing ballast for platform stability. The hard tank 206 includes a water ballast tank 214 at its lower end and a center well 212 located in the center of the hard tank 206. The center well 212 can be filled with water from top to bottom and directly connected with open seawater at the keel of the SPAR platform 100 via a water passageway 218 (or a riser guide tube), which is watertight and extends through the mid tank 208. A watertight deck 220 can be applied at the top (in the center well 212 of the hard tank 206) and the end (at the bottom of the soft tank 210) of the water passageway 218. FIG. 2B is a top view of the watertight deck 220 located at the top of the water passageway 218 in the hard tank 206. The watertight deck 220 can contain multiple watertight deck openings 222 for the water passageway 218 to pass through. Also, a set of mooring lines 216 can be applied to the exterior of the SPAR platform 100 to secure the position of the floating SPAR platform 100 to the seabed (not shown) in the seawater.
In a traditional SPAR platform, such as the SPAR platform 100 shown in FIGS. 1 and 2A, a wet storage system may be applied. The definition of the wet storage is that the oil is stored by displacing the water in the same tank or compartment and an oil-water interface is created in between. The advantage of the wet storage method is that the external water pressure and internal water pressure of the storage tank are substantially balanced so that the shell structure of the oil storage tank can be designed more economically for deep draft vertical oil storage applications. The disadvantage is that the wet storage could cause environmental pollution if the displaced oil-contaminated water is discharged into the open seawater directly without proper treatment. It is conventionally considered difficult to separate a large quantity of oil from water completely without costly specialized equipment which would add significant weight to the platform's topside facilities. To solve the pollution problem and comply with applicable environmental laws, the conventional way to export the produced oil from a SPAR platform is via a deepwater pipeline, instead of storing the oil within the platform and exporting the oil via a shuttle tank. However, the deepwater pipeline is generally very costly to construct because deepwater offshore oil fields are usually very far from shore.
As described above, a need exists for an improved oil storage system to be applied with an offshore oil drilling and production platform.
A further need exists for an improved oil storage system with an improved water displacement method to separate oil from water before water being discharged into open seawater to avoid environmental pollution.
The present embodiments of the system and the method meet these needs and improve on the technology.
The present embodiments are detailed below with reference to the listed Figures.